isogeotherm

from The Century Dictionary.

• noun In physical geography, an imaginary line or surface under the earth's surface passing through points having the same temperature.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Phys. Geog.) A line or curved surface passing beneath the earth's surface through points having the same mean temperature.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A geoisotherm.